1. Field of the Invention
The present invention relates generally to insulator layers and polysilicon resistors formed within integrated circuits.
More particularly, the present invention relates to a structure of insulator layers formed within an integrated circuit, which structure of insulator layers maintains the integrity of a polysilicon resistor formed within that structure of insulator layers.
2. Description of Related Art
In addition to the use of transistors and diodes as switching elements and current rectifying devices in semiconductor integrated circuits, it is also quite common in the art that those circuits will have resistors incorporated into their designs and fabrications. Commonly, a resistor structure within an integrated circuit will provide an electrical load which assures proper operation of the circuit under the electrical current and voltage parameters to which the circuit was designed.
Of the several methods and materials from which resistors may be formed within integrated circuits, the fabrication of resistors from layers of polysilicon formed upon semiconductor substrates has become quite common in the art. Resistors formed from such polysilicon layers possess several advantages within integrated circuit fabrications. First, such resistors are readily and conveniently manufacturable since the polysilicon layers from which polysilicon resistors may be fabricated have several additional applications within integrated circuits. In addition, resistors formed from polysilicon layers may be fabricated through a wide range of resistances by means of incorporating various types and quantities of dopant species into those layers.
Although the fabrication and use of resistors formed from polysilicon layers provides an attractive and convenient method for fabricating resistors within integrated circuits, the fabrication and use of such resistors is not without several problems. One of the more significant problems with the fabrication and use of polysilicon resistors within integrated circuits is the stability of the resistance of the polysilicon layer from which is formed the polysilicon resistor. As is known in the art, the methods by which polysilicon layers are formed upon semiconductor substrates yield polysilicon layers which possess significant residual reactivity. This residual reactivity of the polysilicon layers remains even after the layers have been formed upon a semiconductor substrate, patterned into polysilicon resistors and exposed to several additional integrated circuit processing operations.
The reactivity of these layers is believed to result from incomplete reaction of the precursor materials from which polysilicon layers are formed. Specifically, the incomplete reaction of the precursor materials is believed to yield residual reactive sites within the polysilicon resistor formed from the polysilicon layer. Upon subsequent semiconductor processing operations, various mobile species may infiltrate into the polysilicon resistor and react with the residual reactive sites within the polysilicon resistor causing significant chemical and/or physical changes in the polysilicon resistor material. These chemical and/or physical changes may be manifested by significant changes in the resistivity of the polysilicon resistor. Such changes are often difficult to predict and may lead to significant reliability or functionality problems in semiconductor integrated circuits in which those polysilicon resistors are formed.
It is thus desirable, in order to assure optimal and reproducible performance of polysilicon resistors and the integrated circuits into which those polysilicon resistors are formed, to provide integrated circuit structures surrounding those polysilicon resistors which optimally assure the stability of the resistance of those polysilicon resistors. The stability of the resistance is preferably assured independent of the quantity or nature of integrated circuit processing operations to which those polysilicon resistors may subsequently be exposed.
The present invention is directed towards the goal of assuring the stability of the resistance of polysilicon resistors within integrated circuits independent of subsequent processing operations to which those polysilicon resistors may be exposed. The method by which the present invention achieves that goal is through the choice of insulator layers which surround a polysilicon resistor within an integrated circuit.